[CORS-RFC1918] Extract tests to their own file.

I have promised a few times that I would extract these tests once they
reached a critical mass and I had a minute. That time has come.

Bug: chromium:1167697
Change-Id: I5808dfa7bfac32190c39354895096c9e0679a398
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632681
Commit-Queue: Titouan Rigoudy <titouan@chromium.org>
Auto-Submit: Titouan Rigoudy <titouan@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844712}

content/test/BUILD.gn

@@ -1101,6 +1101,7 @@ test("content_browsertests") {
     "../browser/renderer_host/back_forward_cache_metrics_browsertest.cc",
     "../browser/renderer_host/blocked_scheme_navigation_browsertest.cc",
     "../browser/renderer_host/cookie_browsertest.cc",
+    "../browser/renderer_host/cors_rfc1918_browsertest.cc",
     "../browser/renderer_host/embedding_token_browsertest.cc",
     "../browser/renderer_host/frame_tree_browsertest.cc",
     "../browser/renderer_host/input/autoscroll_browsertest.cc",